how to join two Navigation mesh?
You need to select both of the meshes and select join from the lefthand Tools->Edit->join area.

Once you have done that then you click on the physics tab on the right and select NavMesh Reset Index Values.

if you find you need to join two nav mesh's you probably need to go through and select all your objects, hit CTRL + A and select location, rotation, and scale for all your objects, then rerun the nav mesh generator, it should fix your issues and you won't need to join meshs.